In a fuel injection system having multiple fuel supply components (such as the fuel lances described below), it is sometimes necessary to shut down one or more of the fuel supply components while at least one of the remaining fuel supply components continues to operate. In this case, there is a need to consider purging the fuel supply component or components that have been shut down to remove any residual fuel that may be trapped in the component or components. Otherwise, there is a risk that in some circumstances the residual fuel might mix with air and undergo combustion close to, or within, the fuel supply component or components leading to overheating and damage of the same. Moreover, even in the absence of air, if the residual fuel is subject to elevated temperatures then this can lead to fuel decomposition and the undesirable formation of soot with the potential to block the fuel supply component or components.
In one particular example, a burner is arranged in the plenum of a gas turbine engine and leads with an inner injection space into a combustion chamber. Compressed air is admitted to the burner from the compressor exit plenum of the gas turbine engine. A main fuel supply component of the fuel injection system injects fuel into the air and a fuel lance (sometimes called a pilot fuel lance) is used to periodically supply a gaseous fuel such as methane into the injection space.
The fuel lance has a central bore or passage (normally called the gas pilot channel) for carrying the fuel. In some cases, a number of separate fuel lances are supplied with fuel from a single fuel manifold and a check valve (or non-return valve) prevents the flow of fuel back though the fuel lance and into the fuel manifold. The fuel manifold is supplied with fuel and at least one purge gas such as nitrogen through valves that switch between a fuel supply and a purge gas supply.
There is a risk that ignition of the fuel can take place close to, or inside, the fuel lance under certain conditions. One such condition arises if the fuel lance is not sufficiently purged with nitrogen after a supply of fuel has been completed, or before a supply of fuel is commenced. Another condition arises if fuel trapped in the fuel manifold upstream of the check valve is discharged past the check valve during load decrease driven by the pressure difference between the fuel manifold and the combustion chamber. It will be readily appreciated that both of these conditions depend critically on whether or not the check valve is operating properly and within specified limits.
Because of the risk that the fuel might ignite close to, or inside, the fuel lance, the operating temperature of the fuel lance can be monitored by placing a thermocouple at the tip of the fuel lance and in the gas pilot channel near the outlet of the check valve. However, in practice the use of thermocouples is not entirely satisfactory. First of all, the thermocouples only detect overheating local to the thermocouple and significant damage to the fuel lance can occur before the overheating is detected. The output signals from the thermocouples are not reliable and can cause restriction in the operation of the gas turbine engine. The location of the thermocouples also makes it difficult to access them for maintenance and repair.
Accordingly, there is a need for an improved fuel injection system with means for allowing the effectiveness of any purge sequence and/or the operating condition of the fuel injection system and the check valve to be monitored in a reliable way so that overheating can be prevented.